The present invention relates to storage-stable glyoxalated polyacrylamide, high solids aqueous compositions thereof, and their use for papermaking.
Glyoxalated polyacrylamide (G-PAM) is used in a variety of paper grades to provide paper with dry and temporary wet strength. For example, glyoxalated polyacrylamide increases the initial wet strength of many household tissues which come in contact with water in use. Glyoxalated polyacrylamide is also applied to increase the compression strength and the dimensional stability of many board-grade paper products.
Glyoxalated polyacrylamide generally has been prepared by reacting glyoxal with a cationic polyacrylamide in slightly alkaline aqueous solution and stabilized under acidic condition. Commercially available glyoxalated polyacrylamide products contain a large number of un-reacted amide groups, which can react with glyoxal under storage, leading to the continuing cross-linking of the base polyacrylamide molecules and shortened product shelf life. Also, in practice, the polyacrylamides have contained relatively low amounts of cationic monomer, and typically below about 5 mole percent thereof, limiting the cationic charge contribution of this component.
Glyoxalated polyacrylamide generally can not be dried into a solid particulate form without inducing significant and rapid crosslinking of the glyoxalated polyacrylamide. Glyoxalated polyacrylamide is thus typically stored and transported in a bulk aqueous fluid carrier. The volumes of formulated glyoxalated polyacrylamide transported in this manner can be significant, which can require large volume containers or tanker vessels for transport. Under storage in bulk aqueous fluid, glyoxal still tends to continue cross-linking the base polyacrylamide molecules in prior formulations, ultimately resulting in solution gelling and the loss of performance. To provide glyoxalated polyacrylamide with ample storage stability, commercially available glyoxalated polyacrylamide products ordinarily have been prepared in a dilute concentration of about 7.5%, giving a shelf life of about 24 days. It has been reported that 10% glyoxalated polyacrylamide solutions gel within 8 days at 25° C., and that 7.5% glyoxalated polyacrylamide solutions gel within 24 days at 25° C., such as disclosed by Farley, C., Chapter 3—Glyoxalated Polyacrylamide Resin, in Wet-Strength Resins and Their Applications, Tappi Press, 1994. Further, these storage periods were indicated to be reduced 50% where the storage temperature was increased from 25° C. to 30° C. For the past few decades, researchers have devoted considerable efforts to reducing manufacturing and shipping costs of glyoxalated polyacrylamides. However, reported prior methods have either significantly increased cost or gave poor wet/dry strength performance in paper.
U.S. Pat. No. 3,556,932 relates to glyoxalated polyacrylamide that can be prepared by reacting glyoxal with a cationic polyacrylamide under slightly alkaline conditions. The cationic polyacrylamide (base polymer) has a molecular weight of 7000-25,000 Daltons. The cationic charge is obtained by copolymerizing acrylamide with a cationic monomer, which is typically dimethyldiallylammonium chloride. The cationic monomer dimethyldiallylammonium chloride is exemplified in U.S. Pat. No. 3,556,932 as used in an amount of about 2 mole percent of the acrylamide-diallydimethyl ammonium chloride copolymer.
U.S. Pat. No. 4,605,702 relates to the preparation of glyoxalated polyacrylamide using a base polymer with a low molecular weight ranging from 500 Daltons to 6000 Daltons. The exemplified glyoxalated polyacrylamide products in U.S. Pat. No. 4,605,702 have a concentration of about 20% by weight cationic monomer, but were indicated as being used in much higher dosages to compare wet strength properties with exemplified products in U.S. Pat. No. 3,556,932.
U.S. Pat. No. 4,954,538 relates to the preparation of glyoxalated polyacrylamide microparticles with inverse microemulsion polymerization techniques using surfactants to isolate polymer molecules from each other. U.S. Pat. No. 4,954,538 indicates that commercial glyoxalated acrylamide polymers, supplied as 10% solids solutions, gel within about 8 days at room temperature.
U.S. Pat. No. 5,723,022 relates to compositions comprising a blend of (A) an ionic, water soluble, vinylamide polymer containing sufficient vinylamide and —CHOHCHO substituents to be thermosetting and a ratio of glyoxal substituents to glyoxal-reactive substituents in excess of about 0.02:1.0 and (B) a glyoxalated, cationic, water soluble vinylamide polymer having a molecular weight of about 500 to about 6000, sufficient glyoxal-reactive and glyoxal substituents to be thermosetting and a ratio of glyoxal substituents to vinylamide substituents being in an excess of about 0.1:1.0. A compound (A) is exemplified in U.S. Pat. No. 5,723,022 as containing about 5% by weight or about 2 mole % diallydimethyl ammonium dichloride.
U.S. Pat. Nos. 7,034,087 and 7,119,148 relate to methods for making a storage-stable glyoxalated polyacrylamide in which glyoxal is added to the base polymer in two portions and a scavenger for aldehyde groups is used.
U.S. Pat. No. 4,603,176 relates to temporary wet strength resin polymers comprising glyoxal-capped polyacrylamides including a polar, non-nucleophilic unit which does not cause the resin polymer to become water-insoluble, and a hydrophilic cationic unit which imparts a positive charge to the resin polymer, in certain prescribed molar percentages. These resins, when incorporated into paper products, are reported to provide good dry and initial wet tensile strength together with fast wet tensile decay.
Prior methods for making glyoxalated polyacrylamide products have tended to either increase cost significantly or give poor wet/dry strength performance. Commercial glyoxalated polyacrylamide products continue to be manufactured and shipped in the dilute concentration of 7.5 wt % or less. It would be desirable to increase the shelf life of these products without having to resort to further dilution of the polymer content in carrier solution and/or use cooling means to keep the product chilled during storage to retard gelling phenomenon. There has been an unmet need in the industry for cost-effective, storage stable, high solid glyoxalated polyacrylamide products that can meet performance expectations.